Cathode ray tube



Aug. 10, 1937. B. P. DUDDING ET AL CATHODE RAY TUBE Filed Nov. 30, 1934 Patented Aug. 10, 1937 PATENT OFFICE CATHODE RAY TUBE Bernard Phineas Dudding, Oxhey, and Leslie Connock Jesty, Wembley, England, assignors to The General Electric Company Limited, London, England Application November 30, 1934, Serial No. 755,336

In Great Britain December 19, 1933 4 Claims.

This invention relates to the envelopes of cathode ray tubes for television and like purposes which involve the formation of an image by the motion of the cathode ray. By a cathode ray tube we mean a device (a) comprising means for producing a beam of cathode rays, and (b) comprising means for deflecting the beam sothat in the later part of its path it can be made to travel substantially along any line passing through the apex of some cone and lying within that cone, (0) comprising an envelope of which the portion within the intersection of the envelope with the said cone is substantially a segment of a sphere having a radius not substantially less than the distance of the said portion from the apex of the said cone, and (12) such that the image produced by the cathode ray is viewed through the portion of the envelope mentioned in (c). In the commonest form of cathode ray tube the said portion of the envelope is coated internally with fluorescent material; hereinafter it will be termed for brevity the screen. But it must be understood that it need not actually be a fluorescent screen and that the cathode rays need not actually impinge on it; for in some tubes the fluorescent screen is a memberseparate from the envelope. However, if such tubes possess the four 1 elements (a), (b) (c) (d) above mentioned, then the portion of the envelope defined under (0) will still be termed the screen, and the inven- 0 tion will be applicable to them; for it will appear that the invention is concerned with the shape of the envelope alone.

The whole envelope might conceivably have the shape of the sphere of which the screen is a segment. It would then be very bulky. Accordingly in all but the smallest cathode ray tubes it is customary to make the portion enclosing the deflecting means (b) in the form of a cylinder whose diameter is much less than that of the screen and to join it to the end of the envelope carrying the screen by a portion having the form of the frustrum of a cone roughly similar and similarly placed to the cone mentioned in (c). This invention relates only to tubes resembling these in so far that, between the screen and the deflecting means (b) there is a section of the envelope having a diameter much less than the screen and therefore also a section having the same diameter 50 as the screen. Only tubes having this feature will be hereinafter termed cathode-ray tubes; and

only such tubes raise the problem to which the invention is directed. The part of the envelope between the screen and the said section of the same 55 diameter as the screen will be termed the end portion. Figure 1 of the accompanying drawing should aid in the understanding of these definitions; it shows, approximately to scale, one form of envelope at present in use; the end-portion, screen, and cone are thus marked in Figure 1.

It will further be assumed that no part of the end portion is concave to the exterior and that the thickness of the glass of the end portion is of that degree of uniformity which naturally results from the blowing of the envelope in a mould.

When the cathode ray tube is of considerable size (say of length greater than 45 cm. long and with a screen more than 15 cm. in diameter) there is difficulty in making glass envelopes thin enough to stand the thermal changes to .which they are subjected and yet thick enough to withstand the external atmospheric pressure when evacuated. Sometimes the envelope of large tubes will endure during manufacture and will collapse without Warning during use. Such collapse is disastrous, not only because the tubes are expensive, but because the implosion is very dangerous to by-standers; the glass and"the electrodes are liable to be projected with great force. The object of this invention is to reduce greatly the risk of such collapse by suitable shaping of the envelope.

The region of greatest strain in the glass is in the end portion. It is particularly severe because tension and compression are liable to occur within a short distance of each other. The general stress is, of course, compressive; but part of the compressive stress on the screen and the parts immediately surrounding it are borne by circumierential tension in the corner where the spherical part joins the conical part. This tension may not be wholly counterbalanced by atmospheric compression; part of the glass here may be in tension while all the remainder is compressed. We have found that liability to collapse can be greatly diminished by shaping the envelope so that there is no part in resultant tension, or even by proceeding part of the way in this direction so that the resultant tension is greatly reduced. This can be achieved by rounding the corners at which the dangerous stress occurs and by increasing the radii of curvature here above the values that are at present usual.

According to the invention the radius of curvature of the end portion of a cathode ray tube is everywhere so great that no part of this portion is in substantial tension when the tube is completely evacuated and exposed to atmospheric pressure on the outside. Substantial tension here means a tension so great that its reduction would We have found that the condition according to the invention can generally be attained, and the risk of collapse can be reduced within reasonable bounds, if the ratio of the radius of curvature of the end portion to the radius of curvature of the screen is nowhere less than 1/7.5. In order that tension should be entire]; remoted it is seldom necessary to make the ratio much greater than 1/5;

The increase of this ratio over the values used at present can be secured either by decreasing the radius of curvature of the screen or by increasing the radius of curvature of the part surroundingthe screen. The first course has the disadvantage of introducing distortion of the image; for the image is least distorted when the screen is of least curvature. Moreover, if this course were pursued too far, the condition would cease to be fulfilled and the envelope would fall outside the scope of the invention. On the other hand the second course, pursued alone, necessarily makes the tube bulkier by increasing its maximum diameter. But the increase required turns; out not to'be very serious. It is possible to make cathode ray' tubes according to the invention in which the ratio of the maximum diameter; to the diameter of that part of the screen on which an image can be reproduced without substantial distortion is no greater than 4/3. Preferably this ratio is not exceeded.

' In the above description it has been assumed that-the-curvature-of the screen is uniform. This assumption is not always true. When it is not true the radius of curvature is to be taken for the purpose of this specification as the mean radius of curvature p and'calculated from where 21 is the diameter of the screen and d the greatest distance between any point on it from a plane passing through its boundary.

Figure 2 of the accompanying drawingv shows one example of,.an envelope constructed in accordance with the invention. The radius of curvature of thescreen l. is 28 cm., the radius of; curvature at;.the point: marked 2 is 6 cm.; the diameter, of; that ,part of the screen on which an image can be, reproduced is 23 cm. and the maximum diameter of the tube is 30 cm.

Figure 3 of the accompanying drawing shows another; example. Here the radiusof curvature of the screen I: is 32-cm., the radius of curvature at. the point marked 2 is, 4.8 cm.; the diameter of that partof the screen on which the image can be reproduced is 23.5 cm.; the maximum diameter of the tube is 29.5 cm.

We claim:-

1. An envelope for a cathode ray tube, comprising a main portion, part at least of which is substantially in the form of a. conic frustum, a screen portion having a radius of curvature of 28 cm. and a diameter of 23 cm. and a curved end portion connecting the main portion to the screen portion and having a minimum radius of curvature of 6 cm. the maximum diameter of the envelope being 30 cm.

2. An envelope for a cathode ray tube, an envelope comprising a main portion, part at least of which is substantially in the form of a conic frustum, a screen portion having a radius of curvature of 32 cm. and a diameter of 23.5 cm. and a curved end portion connecting the main portion to the screen portion and having a mini mum radiusof curvature of 4.8 cm. the maximum diameter of the envelope being,29.5 cm.

3. An envelope of vitreous material for a cathode ray tube of large size, comprising a screen portion. formed as a spherical segment of not less than 15 cm. in diameter, a substantially conical portion symmetrical with said screen portion and connected therewith by a connecting portion formed as. a. curved surface of revolution with a radius of not less than 1/'7.5 of the radius of said screen portion, the radius of said screen portion being not less than the distance between the point of intersection of the axis of the tube with the screen and the apex of a virtual cone passing through the periphery of said screen and having its generating line substantially parallel with said conical portion, whereby the tensional stresses in said curved connecting portion when the tube is exhausted shall not exceed a predetermined safe limit for the vitreous material of which the tube is formed.

4. An envelope of vitreous material for a large size'cathode ray tube comprising a screen in the form of a spherical segment, the diameter of which is in excess of 15 centimeters and the spherical radius of which is in excess of said diameter, a substantially frusto-conical portion and an end-portion uniting said screen with said frusto-conical portion, said end portion being in the form of a curved surface of revolution, the ratio'of the radius of the uniting end portion to the spherical radius of the screen being not less than 1/'l.5 so that thetensional stresses in the envelope when exhausted shall not exceed a predetermined safe limit for the vitreous material of the'envelope.

BERNARD PHINEAS DUDDING.

LESLIE CONNOCK' JESTY. 

